My one facebook criticism

I have complimented Facebook many times, and I continue to do so regularly, but here I would like to point out one of the few frustrating things about Facebook.
Over time Facebook has become a great address book, even better than the alumni directory for my university. Whenever I want to get in touch with someone, I can simply click and message them directly from Facebook. That person gets an email notifying them that they've received a Facebook message and that they can log in to read it. The problem here being that Facebook is essentially duplicating e-mail within their own system. Kudos to them for doing this so successfully, and getting so many of their users to use it. I would venture to bet that even some of their users use Facebook messaging more than they use their e-mail, and I am being forced to join in on this.

The question I have is does it make sense to use Facebook instead of e-mail? I understand why Facebook has this service, and why Facebook does not let people export their contacts. It simply drives up the usage of their service. But for people who manage everything out of their e-mail accounts (gmail), this is extremely frustrating, especially since you can't even copy and paste people's e-mails from their Facebook profiles.

Over time my Facebook social network continues to grow, and slowly capture even the most obscure people from my past. It is a slow manual migration from messaging people directly from Facebook ( because for a one off message its just easier) to getting that person's e-mail into my Gmail account. Once that person is in my Gmail account, I don't need to go to Facebook anymore. The separation of my address book and my messaging is frustrating. This Facebook become more like gmail? or does my Gmail (or firefox for that matter) become more like Facebook?